1. Field of the Invention
The present invention is directed to a specific process and a specific apparatus for generating chlorine dioxide directly from aqueous solutions of chloric acid or chloric acid/sodium chlorate mixtures employing an oxygen-evolving catalyst. In particular, the present invention relates to a specific process that employs the chemical reduction of chloric acid with water in the presence of an oxygen-evolving catalyst without the use of or addition of either another acid or an added reducing agent.
Furthermore, the present invention relates to an apparatus for generating chlorine dioxide using a reaction zone containing an oxygen-evolving catalyst that is physically separate, but hydraulically connected to an aqueous chloric acid/chlorine dioxide gas disengagement zone.
2. Brief Description of the Art
Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and in a number of other uses because of its high oxidizing power. There are a number of chlorine dioxide generator systems available in the marketplace. Most of the very large scale generators utilize an alkali metal chlorate salt, a reducing agent, and a strong acid. If sodium chloride is employed as a reducing agent or if hydrogen chloride is employed as the acid, then a mixture of chlorine and chlorine dioxide is produced.
Generally, the additional presence of chlorine in a chlorine dioxide product is not desired and, for that reason, many processes have been developed to produce chlorine dioxide having little or no chlorine concentration therein. These processes use nonchlorine-containing acids such as sulfuric acid and reducing agents such as hydrogen peroxide, methanol or other organic compounds, sulfur dioxide or other sulfur-oxygen species having a sulfur valence of less than +6, nitrogen oxide, nitrogen dioxide, or carbon monoxide and the like.
However, if organic compounds are used as reducing agents in these processes, unreacted volatile organics including formic acid may be present in the chlorine dioxide product. Their presence may be generally undesirable for many applications. If sulfur-containing acids or reducing agents are used, sulfate salts or sulfuric acid may accumulate in the reaction system as undesirable byproducts. If gaseous reducing agents such as sulfur dioxide or carbon monoxide are employed, complex reactor designs and process control systems must be employed to prevent such unreacted gaseous reducing agents from leaving the reaction zone with the chlorine dioxide product.
Furthermore, prior art processes for the production of chlorine dioxide that use alkali metal chlorates and excess acid precursors accumulate as alkali metal salts in the reaction system. This salt accumulation must be periodically removed from the system as an unwanted byproduct, either as a solid or liquid solution. This periodical removal may cause a temporary shutdown of the reaction system as well as the end process that the chlorine dioxide is being used to treat.
Numerous U.S. patents describe processes for generating chlorine dioxide by reacting an alkali metal chlorate, a mineral acid, and a reducing agent. Examples of such U.S. Pat. Nos. included 4,938,943 (Norell); 4,978,517 (Norell et al.); 4,986,973 (Svedin et al.); 5,002,746 (Norell); 5,091,166 (Engstr om et al.); 5,091,167 (Engstr om et al.); and 5,093,097 (Engstr om et al.)
Separately, it is known to generate chlorine dioxide by reacting an aqueous solution of an alkali metal chlorate and a mineral acid such as sulfuric acid or phosphoric acid in the presence of selected catalysts.
For example, U.S. Pat. No. 4,362,707, which issued to Hardee et al. on Dec. 7, 1982, teaches a process for generating chlorine dioxide reacting an alkali metal chlorate and an acid in the presence of catalyst comprising the mixture of valve metal oxide and at least one of ruthenium oxide, iridium oxide, palladium oxide, rhodium oxide, and platinum oxide. Sulfuric acid, hydrochloric acid, and phosphoric acids are the only explicitly named acids for this process. (See col. 4, lines 47-50; col. 6, lines 33-37; and claim 2 of the '707 patent.)
U.S. Pat. Nos. 4,381,290 and 4,501,824, both of which issued to Hardee et al. on Apr. 26, 1983 and Feb. 26, 1985, respectively, teach a process for reacting an alkali metal chlorate with an acid feedstock in the presence of a heterogeneous catalyst that is substantially insoluble in the reactant solutions and is selected from at least one ruthenium oxide, iridium oxide, palladium oxide, rhodium oxide, and platinum oxide. Sulfuric acid, hydrochloric acid, and phosphoric acid are the only explicitly named acids for this process. See col. 4, lines 65-68; col. 6, lines 51-55; and claim 2 of the '290 patent.
Also, it is known to generate chlorine dioxide electrochemically from an aqueous feedstock solution of an alkali metal chlorate and a mineral acid.
U.S. Pat. No. 4,426,263, which issued to Hardee et al. on Jan. 17, 1984, teaches an electrochemical process for generating chlorine dioxide involving electrolyzing the combination of a chlorate-containing feedstock with an aqueous strong acid in an electrolytic cell having an electrocatalytic cathode, including certain platinum group metal oxide mixtures. Sulfuric acid, hydrochloric acid, and phosphoric acid are the only explicitly named acids for this process. (See col. 4, lines 66-68; col. 6, lines 46-50; and claims 1, 2, 3, and 7 of this '263 patent.) The electrochemical cell for this process has an electrocatalytic cathode mode from a platinum group metal oxide mixture selected from a group consisting of ruthenium-rhodium, ruthenium-palladium, rhodium-palladium, iridium-rhodium, iridium-platinum, and ruthenium-rhodium-palladium.
U.S. Pat. No. 4,767,510, which issued to Lipsztajn on Aug. 30, 1988, teaches a process for generating chlorine dioxide by an electrochemical process where an aqueous acidic solution of chlorate ions having a total acid greater than 7 normal sulfuric acid is subjected to a cathodic electrical current. The cathode for this electrolytic cell is constructed of an electrochemically active material which is also chemically inert and noncatalytic with respect to the production of chlorine dioxide.
To avoid the formation of some or all of the above-noted byproducts, it has also been proposed to use chloric acid instead of all or part of the alkali metal chlorate salt precursor for chlorine dioxide generating systems.
For example, see U.S. Pat. Nos. 5,084,148 (Kaczur et al.); 5,174,868 (Lipsztajn et al.); 5,223,103 (Kaczur et al.); 5,242,553 (Kaczur et al.); 5,242,554 (Kaczur et al.); 5,248,397 (Cawlfield et al.); 5,258,105 (Kaczur et al.); 5,264,089 (Kaczur et al.); 5,284,443 (Lipsztajn et al.); 5,296,108 (Kaczur et al.); 5,322,598 (Kaczur et al.); 5,348,683 (Kaczur et al.); and 5,354,435 (Kaczur et al.).
Also, it is known to electrolyze a chloric acid solution to produce chlorine dioxide. See U.S. Pat. No. 5,089,095 (Cawlfield et al.).
Furthermore, it is known to produce chlorine dioxide by heating a reaction mixture comprising an aqueous solution containing hydrogen ions, chlorate ions, and perchlorate ions in the presence of an oxygen-evolving catalyst in solid form in the absence of an added reducing agent. See U.S. Pat. No. 5,342,601 (Cawlfield et al.).
While the chlorine dioxide generating systems disclosed in the above-noted U.S. patents are quite suitable for many commercial applications, there is still a need for a chlorine dioxide generating system that can do all of the following:
(1) can be easily and safely started-up and shutdown; PA1 (2) preferably employs a chemical precursor or precursors that do not generate any byproduct salts or the like that require periodic shutdown of the process; PA1 (3) has a process design that prevents potentially hazardous chlorine dioxide concentrations from accumulating, especially when electric power is lost unexpectedly or shutdown; PA1 (4) has a process design that can introduce the heat required to evaporate water in a way to avoid decomposition of the chlorine dioxide product; PA1 (5) has a process design that both prevents corrosion of the apparatus in the reaction system and avoids the need for costly corrosion resistant materials; PA1 (6) has a process design that utilizes a minimum of moving parts and seals, thereby reducing the potential for leaks of the reactive precursors or the chlorine dioxide product; and PA1 (7) has a process design that can operate at steady state conditions with a minimum number of controls and sensors. PA1 (a) introducing an aqueous chloric acid solution into a reaction zone containing an oxygen PA1 (b) chemically reducing chloric acid with water in said reaction zone in the presence of said oxygen-evolving catalyst and in the absence of both another acid and an added reducing agent, thereby producing a reaction product comprising chlorine dioxide, oxygen, water vapor, and a spent aqueous chloric acid solution; PA1 (c) transferring said reaction product to a disengagement zone; PA1 (d) separating a gas phase comprising chlorine dioxide, oxygen, and water vapor from a liquid phase comprising said spent aqueous chloric acid solution; wherein said reaction zone is physically separated but hydraulically connected to said disengagement zone and said reaction zone immediately drains empty when said aqueous chloric acid solution is no longer introduced, thereby immediately stopping the chemical reduction of said aqueous chloric acid solution. PA1 (a) a source for an aqueous chloric acid solution; PA1 (b) a reaction zone containing an oxygen-evolving catalyst, said reaction zone (i) capable of converting said aqueous chloric acid solution into a reaction product comprising chlorine dioxide, oxygen, water vapor, and a spent aqueous chloric acid solution; and (ii) designed to immediately drain empty when said aqueous chloric acid solution is no longer introduced into said reaction zone, thereby immediately stopping the chemical reduction of said aqueous chloric acid solution; PA1 (c) a conduit for introducing said aqueous chloric acid solution from said source to said reaction zone; PA1 (d) a gas/liquid disengagement zone for separating a gas phase containing chlorine dioxide, oxygen, and water vapor from a liquid phase comprising said spent aqueous chloric acid solution; PA1 (e) a conduit for transferring said reaction product from said reaction zone to said gas/liquid disengagement zone; PA1 (f) a conduit for removing said separated gas phase from said gas/liquid disengagement zone; and PA1 (g) a conduit for removing said separated liquid phase from said gas/liquid disengagement zone.
Despite the seemingly small differences between the process and apparatus of the present invention from those of the prior art, it will be apparent that these differences provide a novel means of chlorine dioxide generation that uniquely provides a safe and inexpensive generator meeting all of these conditions.